GNU tar & incremental backups

[Bill Bogstad]

	A number of people have been questioning whether or not
GNU tar can really be used for incremental file system backups.
Below I provide the GNU documentation for their "-G" option
which seems to fit the bill.  They also have -M (multi-volume) and
-V (volume header) options.

For creation

`-G'
     This option should only be used when creating an incremental backup of
     a filesystem.  When the `-G' option is used, `tar' writes at
     the beginning of the archive an entry for each of the directories that
     will be operated on.  The entry for a directory includes a list of all
     the files in the directory at the time the dump was done, and a flag
     for each file indicating whether the file is going to be put in the
     archive.  This information is used when doing a complete incremental
     restore.

For extraction

`-G'
     The `-G' option means the archive is an incremental backup.
     Its meaning depends on the command that it modifies.

     If the `-G' option is used with `-t', `tar' will list,
     for each directory in the archive, the list of files in that
     directory at the time the archive was created.  This information is
     put out in a format that is not easy for humans to read, but which
     is unambiguous for a program: each filename is preceded by either a
     `Y' if the file is present in the archive, or an `N' if the
     file is a directory, or is not included in the archive.  Each
     filename is terminated by a null character.  The last file is
     followed by an additional null and a newline to indicate the end of
     the data.

     If the `-G' option is used with `-x', then when the entry
     for a directory is found, all files that currently exist in that directory
     but are not listed in the archive *are deleted from the directory*.

     This behavior is convenient when you are restoring a damaged file system
     from a succession of incremental backups: it restores the entire state
     of the file system to that which obtained when the backup was made.
     If you don't use `-G', the file system will probably fill up
     with files that shouldn't exist any more.


					Bill Bogstad
					bogstad at crabcake.cs.jhu.edu